1. Field of the Invention
The present invention relates to searching to acquire a frequency division multiple access system and particularly relates to searching techniques that may be implemented in a dual mode (digital/analog) wireless telephone or other wireless device.
2. Description of the Related Art
In wireless communications technology, user data (e.g., speech, signaling messages, alphanumeric data) modulate a radio frequency signal for transmission and reception between a base station and a mobile unit. The radio spectrum allocated by regulatory authorities for a wireless system is xe2x80x9ctrunkedxe2x80x9d to allow simultaneous use of a spectrum block by multiple units.
Most analog cellular telephone systems are frequency division multiple access (FDMA) systems. In a FDMA system, a base station communicates with plural mobile units in the geographic region covered by the base station""s antenna (often called a xe2x80x9ccellxe2x80x9d ) by utilizing a separate frequency channel for each separate communications link. The number of channels used by the system therefore defines the number of mobile units that can be simultaneously communicating on the FDMA system in any given cell.
For example, in the United States the Advanced Mobile Phone System (AMPS) is a FDMA system that utilizes frequency modulation for providing mobile cellular telephony. Within each cell, AMPS uses a number of frequency channels to communicate with different mobile units. In order to acquire the AMPS system, the mobile unit typically must scan a number of dedicated control channels (DCCHs), such as 21 contiguous DCCHs and then return to the strongest.
Digital wireless communication systems, on the other hand, typically use time division multiple access (TDMA) or code division multiple access (CDMA) to permit a base station to simultaneously communicate with plural mobile units within a given cell. For instance, IS95 CDMA systems use CDMA in conjunction with FDMA while the global system for mobile communications (GSM) uses TDMA in conjunction with FDMA.
In TDMA, data are digitized and compressed to eliminate redundancy, thus decreasing the average number of bits required to be transmitted and received for the same amount of information. The time line of each of the frequency channels used by the TDMA system is divided into xe2x80x9cframesxe2x80x9d and each of the users sharing the common channel is assigned a time slot within the frames. Each user then transmits or receives a burst of data during its assigned time slot and does not transmit or receive during other times. With the exception of delays required by the bursty data transmission, which typically are small enough to be largely unnoticeable, the TDMA system will appear to the users sharing the frequency channel to have provided an entire channel to each user. In GSM, each channel is divided up in time into frames during which eight different users share the channel. A GSM time slot is only 577 xcexcs(microseconds), and each user gets to use the channel for 577 ps out of every 4.615 ms (millisecond) time interval. 577 xcexcs*8=4.615 ms.
Unlike the TDMA system, the CDMA system generally does not separate the multiple users of a common frequency channel using time slices. Rather, in CDMA, multiple users are separated from each other by superimposing a user-specific high-speed code on the data of each user. Because the applied code has the effect of spreading the bandwidth of each user""s transmission, the CDMA system is often called a xe2x80x9cspread spectrumxe2x80x9d system. In the IS-95 CDMA standard, the spreading signal is applied at a rate of 1.2288 Mbps (megabits per second) and consists of a sequence of bits selected from one of 64 different orthogonal waveforms generated using Walsh functions multiplied by a pseudo-random noise (PN) code sequence. Each such Walsh function typically consists of a repeating 64-bit sequence, and thus has a period of 52.083 xcexcs (64 bits/1.2288 Mbps). A different one of the 64 different Walsh functions is utilized for each sub-channel to be included in the frequency channel. At the receiving end, a particular sub-channel can be decoded using the same Walsh function and PN code that was used to encode the sub-channel. When decoded in this manner, the desired sub-channel signal is reproduced and the signals from the other 63 sub-channels are output as low-level noise. As a result, a user can distinguish its code sub-channel from other users"" sub-channels on the same frequency channel.
As indicated above, various wireless systems use FDMA in conjunction with other trunking techniques. However, references to a FDMA system, FDMA channels or the like herein are intended to mean systems that trunk access using FDMA exclusively.
Conventionally, the initial step in attempting to acquire a FDMA system involves tuning the mobile""s receiver to one of the channels used by the FDMA system, receiving transmissions on that channel, calculating a power reading based on the received transmissions, and then repeating these steps for each channel until all channels have been searched or until an adequate power reading has been obtained. Typically, this will require approximately 5 ms to tune the receiver and approximately 5 ms to obtain an accurate power reading, meaning that such a conventional technique requires approximately 10 ms per channel or approximately 210 ms to search all DCCHs in AMPS. During this time, the mobile unit is required to have its receiver turned on and generally cannot be used to receive or transmit for any other purpose.
The present inventor has recognized that such a long search time is particularly problematic in a dual mode digital/analog wireless telephone and can result in audio degradation and even the possibility of a dropped call when such a search is required to be performed during a telephone call. Moreover, even when performed at other times (e.g., during standby or during initial acquisition of a system), such a search can consume additional battery power.
The present invention addresses the foregoing problem by providing techniques for acquiring and/or searching channels in a FDMA system by processing a number of samples of a signal that includes a block of FDMA channels to obtain power estimates for such channels.
Thus, in one aspect, the invention is directed to acquiring/searching channels in a frequency division multiple access (FDMA) wireless system. Initially, a signal that includes a block of FDMA channels is received and filtered (e.g., using an analog filter to bandpass filter the signal) so as to select a frequency band that includes the block of FDMA channels (e.g., 21 contiguous channels, or all DCCH or similar channels used by the subject FDMA system). Next, the filtered signal is sampled to obtain a number of samples, and the samples are processed to obtain a value for each of multiple different frequency bins, such as by using Fast Fourier Transform processing. Based on these values, a measure of power is calculated for several of the FDMA channels.
The invention also addresses the foregoing problems by processing a received signal that includes a block of FDMA channels to obtain measures of various frequency components in the signal and then using at least one of those measures to calculate a power estimate for a FDMA channel.
Thus, in another aspect the invention is directed to acquiring/searching channels in a frequency division multiple access (FDMA) wireless system. Initially, a signal that includes a block of FDMA channels is received and filtered so as to select a frequency band that includes the block of FDMA channels. The filtered signal is then processed to obtain measures of certain frequency components in the filtered signal, and a power estimate for at least one of the FDMA channels is calculated based on at least one of frequency component measures.
By obtaining and processing samples corresponding to multiple FDMA channels in the foregoing manner, the present invention often can significantly reduce the amount of time that the receiver is tuned to the FDMA band. This can significantly reduce power consumption in the case that the mobile unit is awakening from a sleep or idle mode and then returning to a sleep idle mode, and can significantly improve communication quality in the case that the search is performed during communication on an alternate (e.g., digital) wireless system. Moreover, when such a technique is implemented in a dual mode wireless telephone, a wideband receiver capable of receiving the entire block of FDMA channels already exists and, therefore, no additional hardware typically is required.